1. Field of the Invention
The present invention relates to devices and processes for determining the amount of liquid contained in the atmosphere at a given geographic location. More particularly, the present invention relates to apparatus and methods for employing radiometers to passively monitor the radiation received from the atmosphere and for transforming that radiation into data which indicates the amount of liquid water present in the atmosphere from which the detected radiation is emitted. The present invention is useful for weather research, weather forecasting and other monitoring applications.
2. Description of the Prior Art
Devices for determining the water content present in a given volume have typically employed an active transmitter and receiver combination in the past. For instance, U.S. Pat. Nos. 4,812,739 and 4,820,970 by Swanson show such a system for analyzing the water content of crude oil. Sasaki U.S. Pat. No. 4,297,874 determines the water content of sheet material by introducing it to a resonant cavity.
The Stokesberry and Hasegawa article entitled Automatic Digital Microwave Hygrometer, Model II in the May 1976 issue of the Review of Scientific Instruments at pages 556-558 suggests an arrangement for atmospheric humidity detection by introducing air into a captured environment of a resonant cavity.
Others have used active transmitters to survey the different gases present in a sample volume, sometimes using a single transmitter frequency as in U.S. Pat. No. 4,385,516 by Uffelman and others using swept frequencies as in U.S. Pat. No. 4,132,943 by Gournay et al. In Knochel U.S. Pat. No. 4,546,311, measurement of the moisture content of an aqueous solution is obtained by use of an active transmitter which provides a reference signal to an "evaluation arrangement" along with signals received by a probe in the solution.
Passive radiometer systems have found application for diverse measurement purposes. Methane gas is detected by Faulhaber et al U.S. Pat. No. 4,390,785 with an infrared radiometer and a system that splits received infrared signals to develop a reference signal. Seling U.S. Pat. No. 3,167,714 seeks to determine the absolute temperature of a body via a microwave radiometer operable around 35 GHz and a single antenna to sort thermally radiated signals from noise. Fow et al U.S. Pat. No. 3,380,055 sense temperature with a radiometer that varies the frequency pursuant to known absorption rates.
Dual frequency radiometer systems are shown in Haroules et al U.S. Pat. No. 3,737,905 and Mardon et al U.S. Pat. No. 3,911,435. Haroules et al is directed to solar radiation detection. Mardon et al is intended to discriminate between water, metal and other objects by converting the dual frequency signals received at an antenna and then comparing them to develop a difference frequency.
Recent systems for remote measurement of atmospheric vapor and liquid water employ dual frequency microwave radiometers. It is known that the two constituents of liquid and vapor, as well as oxygen, affect the intensity of microwave energy measured by the radiometers. Certainly when measuring vapor one must account for the presence of liquid as well as oxygen although the latter is merely a function of the surface pressure.
However, it has not been appreciated that when measuring liquid, the amount of vapor can readily be estimated by other simpler means along with oxygen as input for the liquid calculation. Because the vapor content of the atmosphere varies very much more slowly than does the liquid content, it is possible to use a single frequency for measuring liquid water. This is particularly true during winter when the vapor amounts are low to begin with.